It has long been recognized that the next evolutionary objective of televsion would be to provide high-definition television pictures to households throughout the United States. Two basic approaches have been put forward for achieving this next objective. The first approach is to depart from the present National Television System Committee standards (NTSC) and to utilize a new transmission protocol for providing high-definition television. The second approach is to provide high-definition television which could be received as a conventional television picture by conventional television receivers operating according to the NTSC standards or which could be received as a high-definition television picture by newly designed receivers without requiring prohibitively large amounts of bandwidth.
The article entitled "Transmission Primaries and Signal Forms" by T. Fugio and K. Kupota, NHK TECHNICAL MONOGRAPH, No. 32, June 1982, notes that two different signal forms have been proposed for high-definition television signal if the first approach is to be used. One is a composite system in which the transmission of the luminance and chrominance signals are frequency-division multiplexed in their spatial and temporal frequency domains as in conventional television signals and a second system in which the luminance and chrominance information is time-division multiplexed every one or two line scan periods.
A system using the second approach is disclosed in my earlier filed application, Ser. No. 462,065. In that system, one TV channel carries the conventional TV signal while high-frequency luminance and high-frequency chrominance information are provided for in a second TV channel. The baseband signal containing the high-frequency components is derived from the lower sideband output of a product modulator whose inputs are taken from an anti-alias filtered source (camera) of wideband luminance information and a local oscillator whose frequency is a multiple of the conventional chrominance subcarrier frequency embedded in the conventional baseband signal for the other channel. The band-limited portions of the high-frequency chrominance information are multiplexed between alternate horizontal lines. Since only half of the necessary chrominance information is being transmitted to the television receiver, the latter must store and reuse the chrominance information for each horizontal line displayed. The storage and reuse of half of the chrominance information for each horizontal line reduces the vertical chrominance resolution. Whereas for many applications this level of vertical chrominance resolution is not objectionable, certain applications can be enhanced by additional vertical chrominance resolution.
The ratio of chrominance information to the luminance information has been analyzed by many commentators. For example, the experimental work described in the book Principles of Color Television, McIlwain and Dean, John Wiley and Son, 1956, pages 81 to 95, indicates that the ratio of luminance to color should be 4 to 1. The article "Worldwide Color Television Standards--Similarities and Differences", Prichard and Gibson, Color Television, IEEE Press, 1983, gives the ratio of luminance to color for the PAL (Phase Alternation Line) and for only the Q signal of NTSC television systems as 5.5 to 1 and 8.4 to 1, respectively.
In light of the foregoing, it can be seen that there exists a need in the encoding and decoding of high-definition TV signals to provide a luminance to chrominance information ratio of approximately 4 to 1.